Make llvm::StringRef to std::string conversions explicit.

This is how it should've been and brings it more in line with
std::string_view. There should be no functional change here.

This is mostly mechanical from a custom clang-tidy check, with a lot of
manual fixups. It uncovers a lot of minor inefficiencies.

This doesn't actually modify StringRef yet, I'll do that in a follow-up.
diff --git a/lldb/source/Plugins/InstrumentationRuntime/MainThreadChecker/InstrumentationRuntimeMainThreadChecker.cpp b/lldb/source/Plugins/InstrumentationRuntime/MainThreadChecker/InstrumentationRuntimeMainThreadChecker.cpp
index 065a85b..4c2173f 100644
--- a/lldb/source/Plugins/InstrumentationRuntime/MainThreadChecker/InstrumentationRuntimeMainThreadChecker.cpp
+++ b/lldb/source/Plugins/InstrumentationRuntime/MainThreadChecker/InstrumentationRuntimeMainThreadChecker.cpp
@@ -178,10 +178,10 @@
       instance->RetrieveReportData(context->exe_ctx_ref);
 
   if (report) {
-    std::string description = report->GetAsDictionary()
-                                  ->GetValueForKey("description")
-                                  ->GetAsString()
-                                  ->GetValue();
+    std::string description = std::string(report->GetAsDictionary()
+                                              ->GetValueForKey("description")
+                                              ->GetAsString()
+                                              ->GetValue());
     thread_sp->SetStopInfo(
         InstrumentationRuntimeStopInfo::CreateStopReasonWithInstrumentationData(
             *thread_sp, description, report));